1. Field of the Invention
The present invention relates to a screw fastening device and a method of judging loosening of screws.
2. Description of the Related Art
A screw fastening device has been known since the past that acquires a screw from a screw supplying device and fastens the acquired screw onto a workpiece serving as a subject.
A screw fastening device such as this performs screw fastening torque control, detection of screw angle and screw lock, and the like when fastening a screw. Reduction in screw fastening failure is achieved through these control and detection operations. In addition, the screw fastening device judges whether or not the screw is tightly seated on the workpiece after fastening the screw, or in other words, whether or not the screw is loose (i.e., screw loosening). That is, the “screw loosening” means that the head (or neck) of a screw is not tightly seated on the placement surface of a workpiece and is located above the placement surface even when the screw fastening is completed. In contrast, the “screw lock” means that a screw is locked and cannot be screwed into the screw hole any more during its screw fastening operation due to irregularities in sizes of threads, oblique insertion of the screw into a screw hole, packed dusts, or others.
The screw loosening is judged based on various criteria, as described, for example, in JP-A-2004-338043. In JP-A-2004-338043, screw loosening is judged based on whether or not the height of a head section of the screw at completion of screw fastening is within a reference range set in advance, with reference to the height of the head section. In addition, screw loosening is judged based on whether or not the amount of movement of a bit from a position at which the screw and a screw hole mesh when the screw is fastened until the screw fastening is completed is within a reference range, with reference to the length of the shaft of the screw.
The screw is manufactured in adherence to standards, such as Japan Industrial Standards. However, the dimensions have allowable error. Therefore, in conventional judgment of screw loosening, the reference range takes into consideration standard values of the dimensions of specific areas of the screw and allowable error for the standard values.
However, when judgment of screw loosening is performed with reference to the height of the head section of the screw as described above, screw loosening may not be correctly judged. For example, when the reference range is set to a range including the maximum value of the allowable error of the head section, a smaller screw manufactured with the allowable error on the minus side may be judged not to be loose if the height is within the reference range, regardless of the screw actually being loose. In addition, when the reference range is set to a range smaller than the maximum value of the allowable error, a larger screw manufactured with the allowable error on the plus side may be judged to be loose because of the height exceeding the reference range, regardless of the screw not actually being loose. This problem may occur even when the dimension of another area, such as the length of the shaft section of the screw, serves as reference.
In other words, in the conventional judgment of screw loosening, the reference range is, by necessity, set to a range capable of absorbing the allowable error of the screw. Therefore, screw loosening that occurs within the range of allowable error is difficult to judge. In other words, in the conventional judgment of screw loosening, the reference range is limited by the allowable error of the screw. Therefore, improving judgment accuracy, or in other words, improving the rate of correct judgment to be a constant value or more is difficult. In addition, individual differences, such as in the shape of the screw hole, occur in the workpieces serving as the subject. Therefore, similar problems occur even when the reference range is set based on standard values of the dimensions of the workpiece.
Furthermore, another method is known regarding the above-described conventional reference range. Specifically, after the screw has been fastened, the size of the head section of the screw or the amount of movement of the bit during the fastening is measured. The reference range is set such that the occurrence rate of screw loosening is statistically less than an allowable value based on the measured data. Therefore, numerous data measurement operations are required to set the reference range. Work load and operation time for setting the reference range increase. Regardless of the increase in work load and operation time, improving the rate of correct judgment in the judgment of screw loosening is difficult, as described above.